Excitatory transmission at central synapses is primarily mediated by the amino acid glutamate acting through postsynaptic ionotropic receptors (Dingledine et al. Pharmacological Review 51:7-61 1999). The N-methyl-D-aspartate receptor (NMDAR) is one such type of ionotropic glutamate receptor (Dingledine et al. Pharmacological Review 51:7-61 1999). NMDARs are multiprotein complexes located at excitatory synapses within the postsynaptic density (PSD) comprised of the core channel subunits together with associated scaffolding and regulatory proteins that control receptor localization, ionic flux through the receptor and downstream signaling events (Scannevin et al. Nature Reviews Neuroscience 1:133-141 2000; Sheng et al. Annual Review of Physiology 62:755-778 2000). NMDAR's are crucial for central nervous system (CNS) development, neuroplasticity and pathophysiology (Dingledine et al. Pharmacological Review 51:7-61 1999; Sheng et al. Science 298:776-780 2002). Multiple factors regulate NMDAR function, including dynamic cycling of protein phosphorylation and dephosphorylation at serine/theronine or tyrosine residues (Wang et al. Nature 369:233-235 1994; Smart Current Opinion in Neurobiology 7:358-367 1997). The Src protein is one such factor that modulates the activity of the NMDARs (Yu et al. Science 275:674-678 1997; Lu et al. Science 279:1363-1368 1998; Yu et al. Nature 396:469-474 1998).
The non-receptor protein tyrosine kinase Src is a ubiquitous enzyme with key roles in diverse development, physiological and pathological processes (Brown et al. Biochim. Biophys. Acta 1287:121-149 1996). Domains identified in Src-the Src homology 3 (SH3) domain, the SH2 domain and the SH1 (catalytic) domain are signature regions that have been used to define highly-conserved protein modules found in a wide variety of signaling proteins (Pawson Nature 373:573-580 1995). In addition to these highly-conserved regions, Src also contains a region of low sequence conservation and unknown function, termed the unique domain.
Src is highly expressed in the CNS, functioning to regulate glutamatergic neurotransmission and synaptic plasticity (Ali et al. Current Opinion in Neurobiology 11:336-342 2001). At glutamatergic synapses, Src modulates the activity of NMDARs (Yu et al. Science 275:674-678 1997; Lu et al. Science 279:1363-1368 1998; Yu et al. Nature 396:469-474 1998). Src represents a point through which multiple signaling cascades from G-protein coupled receptors (Luttrell et al. Journal of Cell Science 115:455-465 2002), Eph receptors (Henderson et al. Neuron 32:1041-1056 2001; Takasu et al. Science 295:491-495 2002; Murai et al. Neuron 33:159-162 2002) and integrins (Lin et al. Journal of Neurophysiology 89:2874-2878 2003; Kramar et al. Journal of Biological Chemistry 278:10722-10730 2003) converge to upregulate NMDAR channel activity, thus mediating essential neuronal excitation. The upregulation of NMDAR activity by Src is necessary for long-term potentiation (LTP) of synaptic transmission at Schaffer collateral-CA1 neuron synapses in the hippocampus (Ali et al. Current Opinion in Neurobiology 11:336-342 2001), the predominant cellular model for learning and memory (Kandel Science 294:1030-1038 2001).
However, abnormal regulation of NMDARs can have numerous pathologic effects; most resulting from the production of nitric oxide, a signaling molecule which mediates excitotoxicity (Dawson et al. Proceedings of the National Academy of Science USA 88:6368 1991). NMDARS mediate ischemic brain injury, as seen, for example in stroke and traumatic injury (Simon et al. Science 226:850 1984). In addition, abnormal NMDAR regulation has been implicated in Alzheimer's disease, Parkinson's disease (Coyle et al. Science 262:689 1993), schizophrenia (Hirsch et al. Pharmacology Biochemistry and Behavior 56(4):797-802 1997), epilepsy (U.S. Pat. No. 5,914,403), glaucoma (US Application 2002 0077322 A1) and chronic pain (Guo et al. Journal of Neuroscience 22:6208-6217 2002).
Although NMDARs are implicated in numerous pathological conditions, non-selective blocking of their function is deleterious, since complete blockade of synaptic transmission mediated by NMDA receptors is known to hinder neuronal survival (Ikonomidou et al. Lancet: Neurology 1:383-386 2002; Fix et al. Experimental Neurology 123:204 1993; Davis et al. Stroke 31:347 2000; Morris et al. Journal of Neurosurgery 91:737 1999).
Additionally, inhibition of Src kinases may also have deleterious results. Since kinases play a part in the regulation of cellular proliferation, they are frequently targeted for the development of new cancer therapies. The majority of these therapies inhibit function of the kinase catalytic domain, which is often highly conserved between distinct kinases. Thus, inhibition of Src in the CNS with a standard kinase inhibitor may cross-react with additional kinases and adversely affect normal neuronal functions.
Considering the above-mentioned deleterious effects resulting from direct blockage of NMDARs and/or indirect inhibition of NMDARs through the use of kinase inhibitors, it is clear that there remains a need in the art for a method of modifying NMDARs which can attenuate downstream NMDAR signaling, without completely blocking, ion-channel activity.